For two decades, scientists have known the biochemical factors that trigger penile erection, but not what's needed to maintain one. Now an article by Johns Hopkins researchers, scheduled to be published this week by the Proceedings of the National Academy of Sciences (PNAS), uncovers the biochemical chain of events involved in that process. The information, they say, may lead to new therapies to help men who have erectile dysfunction. "We've closed a gap in our knowledge," says Arthur Burnett, M.D., professor of urology at Johns Hopkins Medicine and the senior author of the study article. "We knew that the release of the chemical nitric oxide, a neurotransmitter that is produced in nerve tissue, triggers an erection by relaxing muscles that allow blood to fill the penis. We thought that was just the initial stimulus. In our research, we wanted to understand what happens next to enable that erection to be maintained."